At present in rubber, plastics, paper, paint and other industries, calcium carbonate particles having an average particle diameter of not more than 20 .mu.m are effectively used in huge quantities for various purposes. With a view to precluding aggregation of particles and enhancing the affinity of the particles with a substance to which the particles are being added as a filler, these calcium carbonate particles have their surface treated with the acid or sodium salt of fatty acid, resin acid or other similar acid.
This surface treatment is carried out by a method which comprises mixing by agitation an aqueous suspension of calcium carbonate with a surface-treating agent prepared in the form of a solution or emulsion for adsorbing said surface-treating agent on the surface of calcium carbonate particles, subsequently, dehydrating the resultant mixture, drying and pulverizing the mixture and classifying the resultant particles or by a method which comprises directly spraying calcium carbonate powder with a surface-treating agent.
The former method entails many operations such as dehydrating, drying, pulverizing and classifying and the latter method, although simple in terms of operation, is difficult for the surface-treating agent to be uniformly adsorbed on the surface of calcium carbonate particles. To make the matter worse, neither of the two methods is effective in uniformizing the calcium carbonate particle size in the course of surface treatment.